Conventionally, a flip chip method is used for mounting semiconductors. In this method, the face on which electrodes (bumps) are formed of an integrated circuit (IC) chip and the face on which electrodes (electrode pads) are formed of a substrate are provided to face each other. Then, the IC chip bumps and the substrate electrode pads are electrically connected.
In the flip chip method, normally, a liquid thermosetting adhesive referred to as “underfill,” which is introduced into the gap between the semiconductor chip and the substrate, is cured after the electrodes are interconnected. In this way, the electrode interconnected portion is protected from the outside, and further a stress caused by a difference in linear coefficient of expansion between the IC chip and the substrate is absorbed.
In recent years, IC chips have become increasingly miniaturized. Correspondingly, the pitch between adjacent electrodes and the gap between the semiconductor chip and the substrate are becoming increasingly narrower. As a result, when the underfill is introduced into the gap between the IC chip and the substrate using capillary action, voids may develop, or the time it takes to introduce the underfill may become longer.
To address these problems, a so-referred to as first-in method has been proposed (see Patent Literature 1). In this method, a liquid adhesive referred to as non-conductive paste (NCP), or a film-shaped adhesive referred to as non-conductive film (NCF) is coated onto or affixed to the substrate in advance. Thereafter, the resin is cured by thermal pressurization using a thermal pressurization bonder and the like. In this way, the IC chip bumps and the substrate electrode pads are interconnected.
As the IC chips have become miniaturized, copper bumps, which allows for a decrease in the diameter of the bumps, are becoming more popular as the bump material.
As electronic devices have become smaller in size and more complex in their functions in recent years, a configuration in which a plurality of semiconductor chips are mounted in a single semiconductor package has become widely used. As a mounting configuration, a so-called chip-stack technique is widely used. According to the chip-stack technique, chips are three-dimensionally stacked and electrically interconnected by wire bonding. This makes it possible to reduce the size of the semiconductor package compared with the method whereby a plurality of chips are disposed on the substrate two-dimensionally. The chip-stacking employs a paste-like or film-shaped adhesive. However, a paste-like adhesive has high flowability, and is therefore associated with possible electrode contamination. Accordingly, generally, a film-shaped adhesive is used. Meanwhile, as a semiconductor chip interconnect technique to handle further decrease in size and thickness of chip-stack packages, and high speed transmission, a flip chip interconnect technique is gaining attention. Proposed flip chip mounting techniques include an interconnect technique using ultrasonic bonding or anisotropic conductive adhesive (see Patent Literature 2).
The characteristics required for the film adhesive used as NCF include the absence of voids, and excellent electrical connectivity and its high reliability. Other required film characteristics include cracks not being readily caused and high surface flatness. The film adhesive also requires high workability when used as NCF, i.e., ease of handling.
As mentioned above, copper bumps have small diameters. This means that the interconnect strength per bump is low. Accordingly, reliability of electrical interconnection is particularly important. In addition, the miniaturized IC chips means smaller pitches between adjacent electrodes and smaller gaps between the IC and the substrate. Accordingly, if there are voids, a decrease in interconnect strength and short-circuit defect between wires become more likely. Thus, the absence of voids is particularly important.
The film adhesive used as NCF for the purpose of interconnection by soldering is required to contain a flux activator as an indispensable component for performing satisfactory interconnection by soldering. The flux activator is a component which, through reduction of oxide films on solder and a metal surface to be bonded, enhances wettability and increases the interconnect reliability between a semiconductor element and the substrate, for example.
Patent Literature 2 mentioned above does not indicate that the film adhesive being discussed therein contains a flux activator.
In an adhesive film disclosed in Patent Literature 3, a phenolic hydroxyl compound and a carboxylic compound are indicated as examples of a flux activation compound. However, if a film adhesive containing these exemplary compounds as a flux activator is used as NCF, the following problem may be encountered.
When the IC chip bumps and the substrate electrode pads are interconnected by the above-described procedure using NCF, a shorter thermal pressurization time is preferable from the viewpoint of production efficiency. However, if the adhesive film disclosed in Patent Literature 3 is used as NCF, it takes 30 seconds at 235° C., meaning that production efficiency is low.